The invention relates to synthesis of silicon nitride/silicon carbide (Si.sub.3 N.sub.4 /SiC) composite powders via carbothermal reduction of a mixture of silica (SiO.sub.2) and carbon (C) powders, particularly to those composite powders wherein the silicon carbide portion contains an amount of nitrogen, and to resulting composite powders. The invention also relates to densified bodies, including self-reinforced silicon nitride (SRS) bodies, prepared from the composite powders, especially to those densified by pressureless sintering.
A known method of preparing Si.sub.3 N.sub.4 powder is carbothermal synthesis, a reaction of nitrogen with SiO.sub.2 powder contained in an admixture of SiO.sub.2 and carbon (especially powdered carbon). Komeya et al. (U.S. Pat. No. 4,428,916) prepare alpha-Si.sub.3 N.sub.4 (.alpha.-Si.sub.3 N.sub.4) powder in a nonoxidizing atmosphere by adding a seed such as Si.sub.3 N.sub.4 powder, SiC powder or silicon oxynitride (SiON) powder, to the admixture. EP 206,795 teaches that addition of silicon (Si) metal increases .alpha.-Si.sub.3 N.sub.4 yield. Inoue et al. (U.S. Pat. No. 4,514,370) use a Si.sub.3 N.sub.4 seed that is prepared by carbothermal synthesis at a temperature of 1350.degree.-1550.degree. Centigrade (.degree. C.). DT 2,818,545 teaches that .alpha.-Si.sub.3 N.sub.4 seed yields a product with a high .alpha.-Si.sub.3 N.sub.4 content. JP 63-176,301 uses a Si.sub.3 N.sub.4 seed having a beta-Si.sub.3 N.sub.4 (.beta.-Si.sub.3 N.sub.4) content of 5-50 weight percent (wt %), based on seed weight, to prepare Si.sub.3 N.sub.4 powder with a .beta.-Si.sub.3 N.sub.4 content of 1-20 wt %, based on powder weight. JP 80-015,946 discloses use of a carbohydrate, such as sucrose or starch, as a binder for mixed metal oxide powders and calcining the binder to form a carbonaceous residue. Niihara et al. (U.S. Pat. No. 5,134,097) densify a physical mixture of Si.sub.3 N.sub.4 and SiC powders to form a material having a matrix phase wherein SiC grains are located between and within Si.sub.3 N.sub.4 grains and a disperse phase of SiC grains and SiC whiskers. Pyzik et al. (U.S. Pat. Nos. 4,883,776; 4,919,689; 5,021,372; 5,091,347; 5,118,645; and 5,120,328) prepare SRS ceramic bodies from mixtures of Si.sub.3 N.sub.4 powder and other powdered components that help densify the mixture (densification aids), convert .alpha.-Si.sub.3 N.sub.4 to .beta.-Si.sub.3 N.sub.4 (conversion aids) and cause the .beta.-Si.sub.3 N.sub.4 to form elongated whiskers (whisker growth enhancing aids). Pyzik et al. require at least one of each aid.
A second method of preparing Si.sub.3 N.sub.4 powder, known as direct nitridation, reacts silicon (Si) metal with nitrogen. The resulting powder may contain substantial impurity levels. Although hydrofluoric acid may be used to leach out at least some of these impurities, its use tends to lead to high residual fluorine contents.
A third method of preparing Si.sub.3 N.sub.4 powder, known as diimide synthesis, reacts a halogenated silane compound and a nitrogen compound such as ammonia. The resulting powder has a residual chlorine content of at least 10 parts per million (ppm) of powder.